This invention relates to an apparatus for directing the flow of elastomeric material through an extruder head.
In the art of extruding strips of material whether they be plastic or elastomeric, the use of an extruder having a heated barrel and a screw that provides shear energy to the material to be plasticized is well known. As the material is heated it generally converts from a solid pellet or strip form into a strip of plasticized material at the end of the screw tip that projects the material into an extruder head. This extruder head generally has a flow channel comprised of one or more passages or channels that direct the plasticized material through the extruder head to an outlet or discharge die that forms the material into the proper predetermined cross-sectional profile.
Oftentimes the extruder system is of a complex nature providing two or more dissimilar materials to be coextruded. In one example a duplex tread can be made with a top cap material and a lower base material, each material being specifically designed for its application. In even more complex applications, a triplex extruder can be used in which the cap material and base material also have on each lateral extreme a sidewall material that is simultaneously coextruded and bonded to the other two components. All of these materials are projected into an extruder head that directs the materials into a flow channel which assembles and bonds them so that they come out as one or more solid singular pieces. Oftentimes it is desirable to provide adjustable flow restricting members within these channels such that the proper amount of material is provided throughout the extrudate. Such a mechanism is described in U. S. Pat. No. 5,147,195. In that patent they indicate a difficulty exists when converging streams of dissimilar rheological properties that is, for instance of dissimilar viscosities and elasticities. For example, when a melt stream has a high resistance to flow relative to a melt stream which is to be converged, it may be advantageous to provide a heavy edge flow of the high resistant melt stream prior to the convergence by using a suitable contoured stream contacting surface. Similarly, it may be advantageous in other situations to provide a heavy centerflow of one of the melt streams prior to the convergence.
In U. S. Pat. No. 5,147,195, the invention is directed to providing a unique extrusion apparatus that includes a first flow channel and a second flow channel that are separated by a divider member and which converge. A preferred feature of the apparatus is a segmented, flow restricting member that provides an adjustable stream contacting surface. The segmented flow restricting member has a face portion that cooperate within opposing wall of the divider member to form an output gap of one of the flow channels, an adjustment assembly is operatively in communication with the flow restricting member and is employed to adjust the gap. These flow restricting members simply shut off a portion of the flow channel that the plasticized material is passing through, such that a volumetric change can occur in one channel or the other. While these techniques provide an adjustment capability with dissimilar materials such as is common in multi-material components they provide no means for providing a balanced flow of material when it is a homogeneous material. This is particularly troublesome when elastomeric materials are processed. In that case, the material flow is such that as the rubber passes through the flow channel different flow velocities are created across the cross-section of the channel which will result in a mass unbalance in the as-extruded profile of the component as it comes out of the extruder die. What generally happens is that one side of the die will have the material coming out and swelling to a larger area because it has a higher velocity flow rate than the material on the opposite side of the die.
A further problem arises in the simultaneous extrusion of multiple profiles from the same extruder. In this case, it is generally found that the mass output of each profile is not the same, even though the die for each profile is identical. This imbalance of flow between multiple cavities is related to the flow channel, and the amount of imbalance depends on the Theological properties of the elastomeric material, the temperature distribution in the flow channel and flow channel design. Furthermore, the amount of imbalance varies somewhat when the type of material being extruded is changed and also when unavoidable variations in extruder operating conditions occur, such as fluctuations in temperature distribution. In the art, tool makers have to vary the shape and cross-section of the flow channel to obtain equal outputs of each profile. This requires repetitive machining of the flow channel and once done for one particular elastomer material, cannot be adjusted when other types of elastomers are extruded or fluctuations in extruder operating conditions occur.
In order to compensate for these variations in flow velocities within an extruder head, there has been developed a flow channel. The flow channel of an extruder is that portion of the extruder immediately downstream of the screw tip and immediately upstream of the die for forming the profile for the extruded component. These flow channels are specifically designed to insure that uniform flow of material occurs and that the distribution of the material is generally uniform across the flow channel as the material approaches the die. In order to achieve this the toolmaker often times has to vary the shape and cross-section of the flow channel to insure that the velocity profiles approach the die in as uniform a fashion as possible. Once the flow velocities are optimized so that they are generally uniform across the face of the die, the flow channel is said to be balanced for that particular material. In practice, this requires multiple machinings of flow channels and adjustments of dies in order to achieve this balanced flow. These dies are somewhat dependent on the material being extruded, its basic rheological properties, its temperature and the velocity at which the material is being extruded. All of these engineering factors means that a tremendous amount of tuning is required to create a proper flow channel for a given material being extruded and die construction.
A secondary problem that also relates to the mass and velocity imbalance across the die is the unwanted curvature of the extrudate after it leaves the die, such that instead of obtaining a straight strip, a xe2x80x9cbananaxe2x80x9d-shaped curved strip is obtained. This problem is also somewhat related to the velocity distribution in the material as it flows through the flow channel. If one can visualize an extrudate coming out of a die and it was a flat sheet, the material along the lateral edges of the die may be moving at different velocities such that one side of the extruded material will tend to bow or bend as the other side is moving at a faster velocity, the slower side tending to stay close to the die while the faster moving part is moving quicker away from the die. The resulting effect is a xe2x80x9cbananaxe2x80x9d shaped curvature of the profiled component. This curvature as the component is formed is an indication that the velocities of the material are dissimilar from one side of the die to the other even though the dimensional characteristics of the profile component may seem accurate. This non-uniform velocity change causes the component to have a natural bow. In the preparation of tire treads for example, this effect can have some detrimental effect on the product quality of the resultant tire since the accuracy with which the tread can be applied to the unvulcanized tire is reduced and an asymmetry in the molded tire, called conicity, can be created. It is, therefore, an object of the present invention to provide a flow channel that provides a simple way of balancing the flow between multiple cavities and within each cavity so that the proper mass balance of the formed component is achieved. It is also an objective of the present invention to provide an adjustable means to compensate for mass variation and conicity variation within a profiled component. While these features are somewhat related, each can be provided in separate flow channels or can be used in combination to achieve a proper flow channel for the formation of elastomeric strips. In one application of the invention, the entire inventive concept is directed to an adjustable mass balance feature provided within a flow channel. In the second related application, the flow channel is provided with an adjustable conicity weir for insuring that the profile component is provided in such a fashion that the flow velocities do not create any conicity imbalances. While each of these features may be used separately within a flow channel and have separate utility in and of themselves, it is believed preferable that they be used in combination for an optimal result. In the present application, the mass balance feature will be described in detail with the conicity weir feature being provided in a separate, but related application filed simultaneously with the present application.
Where the present invention works well in balancing and providing improved conicity of an extruded strip of elastomeric material coming out of a die with a single opening, it has been noted that these features are most beneficial when multiple components are produced from a single die having multiple openings for producing two or more strips of material. In these cases it will be easily appreciated that mass balancing and conicity problems are exaggerated because the flow is divided within the flow channel creating two separate flow streams that in order to produce equal or duplicate products must have the balance matched perfectly and the conicity adjusted to insure that flow velocities are uniform or as uniform as possible through each of the die openings forming the strips.
These objectives are achieved by the invention as described below.
An extruder flow channel for simultaneously forming elastomeric strips of predetermined cross-sectional profiles is described. The flow channel has a flow inlet end of a predetermined cross-sectional area Ai and a first and a second flow passage communicating with the flow inlet end and each passage having a flow outlet end of a predetermined cross-sectional area Ao and a flow dam interposed between and separating the first flow passage and the second flow passage. The flow dam has a flow splitter. The flow splitter locally increases or locally decreases the flow area of the first flow passage relative to the flow area of the second flow passage. It will be understood by those versed in the art that more than two extrudates can be made simultaneously by extending the concept as follows: Four extrudates, for example, may be made by a flow channel in which each of the two passages described above is subdivided by dams having flow splitters to create four separate passages.
Returning to the example with two flow passages, in use, the flow splitter is asymmetrically oriented relative to the flow inlet end. This asymmetric orientation effects a difference in mass flow of the elastomeric strip in the first channel relative to the second channel. Preferably the extruder flow channel has a mass flow sensor in each first and second flow passage or on each elastomeric strip.
Most preferably the extruder head flow channel has a flow splitter adjustment mechanism attached to the flow splitter. The flow splitter adjustment mechanism moves the flow splitter during the operation of the extruder to effect a relative change in the flow area of the first flow channel relative to the flow area of the second flow channel. The extruder flow channel preferably has the mass flow sensor in each first and second flow channel, a logic circuit connected to the sensors to measure the difference in mass flow and to calculate the amount and direction the mass flow splitter must move to equalize flow, a means for signaling the amount of flow splitter adjustment, an actuator for moving the flow splitter adjustment mechanism by a predetermined amount requested by the means for signaling. Preferably, the mass flow sensors are located down stream of the mass flow splitter. Additionally, the mass flow sensors signal total mass flow values in each channel to the logic circuit which can calculate the total mass flow to a means for signaling the extruder to increase or decrease extrudate flow. In this way, not only can mass flow differences between the first and second channels be detected, but also the delivery amount of extrudate can be adjusted.
In the preferred embodiment the mass flow splitter has a substantially triangular cross-section having an apex. The apex is in proximity to the flow inlet end, preferably in very close proximity to the flow inlet end. The mass flow splitter is preferably pivotally movable relative to the dam.
In the preferred embodiment each first and second flow passage tapers inwardly in one direction and widens in another direction below the inlet end to the outlet end forming a substantially trapezoidal portion adjacent the outlet end. In each of the trapezoidal portions, there is included a flow weir in each flow passage. In an example, the weir has a substantially triangular shape extending to a height less than the full depth of the channel. Preferably the weirs height reaches a peak of at least 50% of the flow channel depth and is provided to effect a redistribution of the elastomeric flows mass prior to entering the final shaping die of the extruder head.